Evolution of Coastal Landforms: Investigating Sediment Dynamics, Hydrodynamics, and Vegetation Dynamics
Coastal ecosystems provide a wide range of services including protecting the mainland from the destructive effects of storms, nutrient cycling, water filtration, nurseries for fish and crustaceans, and carbon sequestration. These zones are threatened by human impacts and climate change through more frequent intense storms and sea level rise with a projected increase of up to 16 mm/yr for the last two decades of the 21st century. However, it is not fully understood what mechanisms control the formation and degradation of these landforms, and determine their resilience to environmental change. In this work, I highlight the role of various physical characteristics and environmental parameters that contribute to the formation and stability of coastal environments.
First, I develop and use remote sensing analyses to quantitatively characterize coastal dune eco-topographic patterns by simultaneously identifying the spatial distribution of topographic elevation and vegetation biomass in order to understand the coupled dynamics of vegetation and coastal dunes. LiDAR-derived leaf area index and hyperspectral-derived normalized difference vegetation index patterns yield vegetation distributions at the whole-system scale which are in agreement with each other and with field observations. LiDAR-derived concurrent quantifications of biomass and topography show that plants more favorably develop on the landward side of the foredune crest and that the foredune crestline marks the position of an ecotone, which is interpreted as the result of a sheltering effect sharply changing local environmental conditions. The findings reveal that the position of the foredune crestline is a chief ecomorphodynamic feature resulting from the two-way interaction between vegetation and topography.
Next, to shed light on the vertical depositional dynamics of salt marshes in response to sea level rise, I investigate the hypothesis that competing effects between biomass production and aeration/decomposition determine an approximately spatially constant contribution of soil organic matter (SOM) to total accretion. I use concurrent observations of SOM and decomposition rates from marshes in North Carolina. The results are coherent with the notion that SOM does not significantly vary in space and suggest that this may be the result of an at least partial compensation of opposing trends in biomass productivity and decomposed organic matter. The analyses show that deeper soil layers are characterized by lower decomposition rates and higher stabilization factors than shallower layers, likely because of differences in inundation duration. However, overall, decomposition processes are sufficiently rapid that the labile material in the fresh biomass is completely decomposed before it can be buried and stabilized. The findings point to the importance of the fraction of initially refractory material and of the stabilization processes in determining the final distribution of SOM within the soil column.
Finally, I develop a process-based model to evaluate the relative role of watershed, estuarine, and oceanic controls on salt marsh depositional/erosional dynamics and define how these factors interact to determine salt marsh resilience to environmental change at the estuary scale. The results show that under some circumstances, vertical depositional dynamics can lead to transitions between salt marsh and tidal flat equilibrium states that occur much more rapidly than marsh/tidal flat boundary erosion or accretion could. Additionally, the analyses reveal that river inputs affect the existence and extent of marsh/tidal flat equilibria by both modulating exchanges with the ocean (by partially “filling” the basin) and by providing suspended sediment.
sea level rise
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